In normal tissue, cellular proliferation is generally restricted to cells that are required to replenish the tissue. Once cells have terminally differentiated, they have a specialized function and no longer divide. Most tissues are made up of non-dividing cells. Thus, normal cell proliferation is tightly controlled to ensure that only the necessary cells divide. There is also a careful balance between cell division and programmed cell death (apoptosis).
Cell division, sometimes referred to as the cell cycle, has four phases: G1 phase (synthesis of various enzymes required for DNA replication), S phase (DNA replication producing two identical sets of chromosomes), G2 (significant protein synthesis, including production of microtubules) and M phase (nuclear division, cytoplasmic division and formation of new cell membrane). Cell division also includes a complex system of cell signaling networks that allow cells to interpret information from numerous extracellular signals, including through receptor proteins, inflammatory factors and pro-apoptotic and anti-apoptotic signals. Dysfunctional signals include those from genetic mutation, infection, exposure to environmental factors including toxins, system stress, autoimmune disorders, and inflammation.
A range of disorders can occur when the process of cell proliferation becomes dysfunctional, including benign growths, neoplasms, tumorigenesis, cancerogenesis, autoimmune disorders, inflammatory disorders graft-versus-host rejection, and fibrotic disorders.
A number of broad-spectrum anti-neoplastic agents have been developed. Cytoskeletal drugs like paclitaxel target tubulin to arrest mitotic cell division and are used to treat a variety of cancers including ovarian, breast, lung, pancreatic, and testicular tumors (See e.g., Jordan, Wilson, Nature Reviews Cancer (2004) 4: 253-265). Organometallic-based drugs such as cisplatin have been used to treat lymphomas, sarcomas, germ cell tumors, and some carcinomas including bladder, small cell lung cancer, and ovarian cancer. Cisplatin has the ability to bind nitrogenous bases and cause extensive DNA cross-linking that ultimately leads to apoptosis (See e.g., Siddick, Oncogene (2003) 22: 7265-7279). Intercalating and alkylating agents have also been extensive use in the clinic for the treatment of various neoplasms, however, the global toxicity associated with these drugs presents a critical concern for patients requiring long-term therapy.
Palbociclib (PD-033299; Ibrance) is sold by Pfizer for the treatment of estrogen-positive, HER2-negative breast cancer in combination with letrozole. The compound inhibits CDK4 and CDK6. The structure of palbociclib is:

Abemaciclib (LY2835219) is a CDK 4/6 inhibitor currently in human clinical trials for the treatment of various types of cancers. It is in a phase III trial for stage IV non-small cell lung carcinoma; in combination with Fulvestrant for women with breast cancer; and with either anastrozole or letrozole for first line treatment of breast cancer. The structure of abemaciclib is:

Ribociclib (Lee011; Kisqali), is a CDK 4/6 inhibitor approved for use in combination with an aromatase inhibitor to treat some metastatic breast cancers, and is in clinical trials for the treatment of certain other tumors. The structure of ribociclib is:

Various other pyrimidine-based agents have been developed for the treatment of hyperproliferative diseases. U.S. Pat. Nos. 8,822,683; 8,598,197; 8,598,186; 8,691,830; 8,829,102; 8,822,683; 9,102,682; 9,499,564; 9,481,591; and 9,260,442, filed by Tavares and Strum and assigned to G1 Therapeutics describe a class of N-(heteroaryl)-pyrrolo[3,2-d]pyrimidin-2-amine cyclin dependent kinase inhibitors including those of the formula (with variables as defined therein):

U.S. Pat. Nos. 9,464,092, 9,487,530, and 9,527,857 which are also assigned to G1 Therapeutics describe the use of the above pyrimidine-based agents in the treatment of cancer.
WO 2013/148748 (U.S. Ser. No. 61/617,657) titled “Lactam Kinase Inhibitors”, WO 2013/163239 (U.S. Ser. No. 61/638,491) titled “Synthesis of Lactams” and WO 2015/061407 filed by Tavares and also assigned to G1 Therapeutics describes the synthesis of N-(heteroaryl)-pyrrolo[3,2-d]pyrimidin-2-amines and their use as lactam kinase inhibitors.
Other publications include the following. WO 2014/144326 filed by Strum et al. and assigned to G1 Therapeutics describes compounds and methods for protection of normal cells during chemotherapy using pyrimidine-based CDK4/6 inhibitors. WO 2014/144596 filed by Strum et al. and assigned to G1 Therapeutics describes compounds and methods for protection of hematopoietic stem and progenitor cells against ionizing radiation using pyrimidine-based CDK4/6 inhibitors. WO 2014/144847 filed by Strum et al. and assigned to G1 Therapeutics describes HSPC-sparing treatments of abnormal cellular proliferation using pyrimidine-based CDK4/6 inhibitors. WO 2014/144740 filed by Strum et al. and assigned to G1 Therapeutics describes highly active anti-neoplastic and anti-proliferative pyrimidine-based CDK 4/6 inhibitors. WO 2015/161285 filed by Strum et al. and assigned to G1 Therapeutics describes tricyclic pyrimidine-based CDK inhibitors for use in radioprotection. WO 2015/161287 filed by Strum et al. and assigned to G1 Therapeutics describes analogous tricyclic pyrimidine-based CDK inhibitors for the protection of cells during chemotherapy. WO 2015/161283 filed by Strum et al. and assigned to G1 Therapeutics describes analogous tricyclic pyrimidine-based CDK inhibitors for use in HSPC-sparing treatments of RB-positive abnormal cellular proliferation. WO 2015/161288 filed by Strum et al. and assigned to G1 Therapeutics describes analogous tricyclic pyrimidine-based CDK inhibitors for use as anti-neoplastic and anti-proliferative agents. WO 2016/040858 filed by Strum et al. and assigned to G1 Therapeutics describes the use of combinations of pyrimidine-based CDK4/6 inhibitors with other anti-neoplastic agents. WO 2016/040848 filed by Strum et al. and assigned to G1 Therapeutics describes compounds and methods for treating certain Rb-negative cancers with CDK4/6 inhibitors and topoisomerase inhibitors.
WO 03/062236 identifies a series of 2-(pyridin-2-ylamino-pyrido[2,3]pyrimidin-7-ones for the treatment of Rb positive cancers that show selectivity for CDK4/6, including 6-acetyl-8-cyclopentyl-5-methyl-2-(5-piperazin-1-yl-pyridin-2-ylammino)-8H-pyrido-[2,3-d]-pyrimidin-7-one (PD0332991), which was given fast-track approval by the FDA and is currently sold as Ibrance (Palbociclib) by Pfizer for the treatment of metastatic breast cancer.
VanderWel et al. describe an iodine-containing pyrido[2,3-d]pyrimidine-7-one (CKIA) as a potent and selective CDK4 inhibitor (see VanderWel et al., J. Med. Chem. 48 (2005) 2371-2387).
WO 2010/020675 filed by Novartis AG describes pyrrolopyrimidine compounds as CDK inhibitors. WO 2011/101409 also filed by Novartis describes pyrrolopyrimidines with CDK 4/6 inhibitory activity.
Johnson et al. reported that pharmacological inhibition of CDK4/6 using the CDK4/6 inhibitors 6-acetyl-8-cyclopentyl-5-methyl-2-(5-piperazin-1-yl-pyridin-2-ylammino)-8H-pyrido-[2,3-d]-pyrimidin-7-one (PD0332991) and 2-bromo-12,13-dihydro-5H-indolo[2,3-a]pyrrolo[3,4]carbazole-5,6-dione (2BrIC) exhibited IR protective characteristics in CDK4/6-dependent cell lines. (Johnson et al. Mitigation of hematological radiation toxicity in mice through pharmacological quiescence induced by CDK4/6 inhibition. J Clin. Invest. 2010; 120(7): 2528-2536).
There remains a need for additional compounds to treat disorders associated with abnormal cellular proliferation, including a tumor or cancer.